Substrate and Patterning Device for Use in Metrology, Metrology Method and Device Manufacturing Method

ABSTRACT

A pattern from a patterning device is applied to a substrate by a lithographic apparatus. The applied pattern includes product features and metrology targets. The metrology targets include large targets and small targets which are for measuring overlay. Some of the smaller targets are distributed at locations between the larger targets, while other small targets are placed at the same locations as a large target. By comparing values measured using a small target and large target at the same location, parameter values measured using all the small targets can be corrected for better accuracy. The large targets can be located primarily within scribe lanes while the small targets are distributed within product areas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) to U.S.Provisional Application 61/467,080, filed Mar. 24, 2011, which isincorporated by reference herein in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to methods and apparatus for metrologyusable, for example, in the manufacture of devices by lithographictechniques and to methods of manufacturing devices using lithographictechniques. Methods of measuring overlay are described, as a particularapplication of such metrology.

2. Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes, which are often used to measurecritical dimension (CD), and specialized tools to measure overlay, theaccuracy of alignment of two layers in a device. Recently, various formsof scatterometers have been developed for use in the lithographic field.These devices direct a beam of radiation onto a target and measure oneor more properties of the scattered radiation—e.g., intensity at asingle angle of reflection as a function of wavelength; intensity at oneor more wavelengths as a function of reflected angle; or polarization asa function of reflected angle—to obtain a “spectrum” from which aproperty of interest of the target can be determined. Determination ofthe property of interest may be performed by various techniques: e.g.,reconstruction of the target structure by iterative approaches such asrigorous coupled wave analysis or finite element methods; librarysearches; and principal component analysis.

The targets used by conventional scatterometers are relatively large,e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spotthat is smaller than the grating (i.e., the grating is underfilled).This simplifies mathematical reconstruction of the target as it can beregarded as infinite. By placing the target in amongst the productfeatures, it is hoped to increase accuracy of measurement because thesmaller target is affected by process variations in a more similar wayto the product features and because less interpolation may be needed todetermine the effect of a process variation at the actual feature site.Unfortunately, the nature of the small targets and the techniquesrequired to measure them tends to limit the accuracy with which overlayand other parameters can be made, making it lower than can be achievedwith conventional, larger targets. Therefore the benefit of measuringoverlay or other parameters within product areas is not fully realized,because the accuracy of the small target measurements themselves isimpaired.

SUMMARY

Therefore, what is needed is a system and method for present inventionusing small target metrology to measure parameters for example atlocations within product areas on a semiconductor substrate, with anaccuracy similar to that associated with larger targets.

According to a first aspect of the present invention, there is provideda substrate having one or more product features formed on it anddistributed over the substrate, and a plurality of metrology targetsadapted for use in measuring a parameter of performance of alithographic process by which the product patterns have been applied tothe substrate. The metrology targets include a first set of targetsdistributed at locations across the substrate and a second set oftargets which are for measuring the same parameter but which are smallerthan the first set. The second set of targets including a first subsetdistributed at locations between the first set of targets, and a secondsubset distributed substantially at the same locations as the targets ofthe first set.

The parameter may be overlay, and each target may be an overlay gratingformed in two patterned layers on the substrate. In some embodiments ofthe present invention, the product features are arranged in productareas separated by scribe-lanes, and the targets of the first set arelocated primarily within the scribe-lanes, while the targets of thesecond set are distributed within product areas. The targets of thesecond set may be more numerous than those of the first set.

The expression “product features” in the present disclosure is notintended to be limited to product features in their final form in afunctional, manufactured device, but includes precursors of suchfeatures, for example portions of photo-sensitive resist material thathave been exposed to record a pattern, prior to development, etchingetc. that will turn the pattern into physical product features. Inmeasuring overlay between two layers, for example, physical productfeatures that have been etched into an underlying layer may be comparedwith product features that exist as a latent image or in a developedform in a resist layer, prior to forming the functional features thatwill be present in a finished semiconductor device or other productbeing manufactured.

The present invention further provides a method of measuring a parameterof performance of a lithographic process by which product features havebeen applied to a substrate, the method comprising simultaneously withapplying the product features to the substrate, applying a plurality ofmetrology targets, the metrology targets including a first set oftargets distributed at locations across the substrate and a second setof targets which are for measuring the same parameter but which aresmaller than the first set, the second set of targets including a firstsubset distributed at locations between the first set of targets, and asecond subset distributed substantially at the same locations as thetargets of the first set, illuminating the targets and detectingradiation diffracted or reflected by the targets and processing theradiation to determine values for the parameter at the locations of aplurality of the targets in each set, correcting parameter valuesmeasured using the first subset of the second set of targets based on acomparison between values measured at one or more locations using atarget of the second subset and a target of the first set.

The present invention further provides a device manufacturing methodcomprising transferring a functional device pattern from a patterningdevice onto a substrate using a lithographic process whilesimultaneously transferring a metrology target pattern to the substrate,measuring the metrology target pattern to determine a value for one ormore parameters of the lithographic process, and applying a correctionin subsequent operations of the lithographic process in accordance withthe results of the metrology, wherein the metrology target patterncomprises a first set of targets distributed at locations across thesubstrate and a second set of targets which are for measuring the sameparameter but which are smaller than the first set, the second set oftargets including a first subset distributed at locations between thefirst set of targets, and a second subset distributed substantially atthe same locations as the targets of the first set.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present invention,are described in detail below with reference to the accompanyingdrawings. It is noted that the present invention is not limited to thespecific embodiments described herein. Such embodiments are presentedherein for illustrative purposes only. Additional embodiments will beapparent to persons skilled in the relevant art(s) based on theteachings contained herein.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the relevant art(s) to makeand use the invention.

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe present invention.

FIG. 2 depicts a lithographic cell or cluster according to an embodimentof the present invention.

FIGS. 3A-3C show a schematic diagram of a dark field scatterometer foruse in measuring targets according to embodiments of the presentinvention, a detail of diffraction spectrum of a target grating for agiven direction of illumination and a set of four illumination aperturesuseful for providing four illumination modes in using the scatterometerfor diffraction based overlay measurements.

FIG. 4 depicts a known form of small target and an outline of ameasurement spot on a substrate.

FIGS. 5A and 5B depict images of the targets of FIG. 4 obtained in thescatterometer of FIG. 3 in −1^(st) order and +1^(st) order diffraction.

FIG. 6 depicts a known form (large) target and an outline of ameasurement support on a substrate.

FIGS. 7A-7C depict pupil images of the target of FIG. 6 obtained in thescatterometer of FIG. 3 in −1^(st) order +1^(st) order diffraction.

FIG. 8 illustrates general form of a patterning device having productareas, scribe lane areas and metrology targets in both the scribe laneand product areas.

FIG. 9 illustrates more detail of an embodiment of the patterning deviceof FIG. 6, according to the present invention.

FIG. 10 illustrates the principle of combining measurement resultsobtained from large and small targets formed on a substrate exposedusing the patterning device of FIG. 9 to obtain more accuratemeasurements, in an embodiment of the present invention; and

FIG. 11 is a flow chart illustrating a metrology method according to anembodiment of the present invention.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION

This specification discloses one or more embodiments that incorporatethe features of this invention. The disclosed embodiment(s) merelyexemplify the invention. The scope of the invention is not limited tothe disclosed embodiment(s). The invention is defined by the claimsappended hereto.

The embodiment(s) described, and references in the specification to “oneembodiment”, “an embodiment”, “an example embodiment”, etc., indicatethat the embodiment(s) described may include a particular feature,structure, or characteristic, but every embodiment may not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases are not necessarily referring to the same embodiment.Further, when a particular feature, structure, or characteristic isdescribed in connection with an embodiment, it is understood that it iswithin the knowledge of one skilled in the art to effect such feature,structure, or characteristic in connection with other embodimentswhether or not explicitly described.

Embodiments of the invention may be implemented in hardware, firmware,software, or any combination thereof. Embodiments of the invention mayalso be implemented as instructions stored on a machine-readable medium,which may be read and executed by one or more processors. Amachine-readable medium may include any mechanism for storing ortransmitting information in a form readable by a machine (e.g., acomputing device). For example, a machine-readable medium may includeread only memory (ROM); random access memory (RAM); magnetic diskstorage media; optical storage media; flash memory devices; electrical,optical, acoustical or other forms of propagated signals (e.g., carrierwaves, infrared signals, digital signals, etc.), and others. Further,firmware, software, routines, instructions may be described herein asperforming certain actions. However, it should be appreciated that suchdescriptions are merely for convenience and that such actions in factresult from computing devices, processors, controllers, or other devicesexecuting the firmware, software, routines, instructions, etc.

Before describing such embodiments in more detail, however, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a patterningdevice support or support structure (e.g., a mask table) MT constructedto support a patterning device (e.g., a mask) MA and connected to afirst positioner PM configured to accurately position the patterningdevice in accordance with certain parameters; two substrate tables(e.g., a wafer table) WTa and WTb each constructed to hold a substrate(e.g., a resist coated wafer) W and each connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., including one or more dies) of the substrate W. Areference frame RF connects the various components, and serves as areference for setting and measuring positions of the patterning deviceand substrate and of features on them.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus illustrated for the sake of example is of atype having two (dual stage) or more substrate tables WTa and WTb(and/or two or more mask tables). In such “multiple stage” machines theadditional tables may be used in parallel, or preparatory steps may becarried out on one or more tables while one or more other tables arebeing used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask tableMT), and is patterned by the patterning device. Having traversed thepatterning device (e.g., mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor 1F (e.g., an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WTa or WTb can bemoved accurately, e.g., so as to position different target portions C inthe path of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g., mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe patterning device support (e.g., mask table) MT may be realized withthe aid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which form part of the first positioner PM.Similarly, movement of the substrate table WTa or WTb may be realizedusing a long-stroke module and a short-stroke module, which form part ofthe second positioner PW. In the case of a stepper (as opposed to ascanner) the patterning device support (e.g., mask table) MT may beconnected to a short-stroke actuator only, or may be fixed.

Patterning device (e.g., mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment markers may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. The alignment system, whichdetects the alignment markers is described further below.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support (e.g., mask table) MT andthe substrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e., a single static exposure). The substratetable WTa is then shifted in the X and/or Y direction so that adifferent target portion C can be exposed. In step mode, the maximumsize of the exposure field limits the size of the target portion Cimaged in a single static exposure.

2. In scan mode, the patterning device support (e.g., mask table) MT andthe substrate table WTa are scanned synchronously while a patternimparted to the radiation beam is projected onto a target portion C(i.e., a single dynamic exposure). The velocity and direction of thesubstrate table WTa relative to the patterning device support (e.g.,mask table) MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the patterning device support (e.g., mask table) MTis kept essentially stationary holding a programmable patterning device,and the substrate table WTa is moved or scanned while a pattern impartedto the radiation beam is projected onto a target portion C. In thismode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after eachmovement of the substrate table WT or in between successive radiationpulses during a scan. This mode of operation can be readily applied tomaskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Lithographic apparatus LA is of a so-called dual stage type which hastwo substrate tables WTa, WTb and two stations—an exposure station and ameasurement station—between which the substrate tables can be exchanged.While one substrate on one substrate table is being exposed at theexposure station, another substrate can be loaded onto the othersubstrate table at the measurement station and various preparatory stepscarried out. The preparatory steps may include mapping the surfacecontrol of the substrate using a level sensor LS and measuring theposition of alignment markers on the substrate using an alignment sensorAS. This enables a substantial increase in the throughput of theapparatus. If the position sensor IF is not capable of measuring theposition of the substrate table while it is at the measurement stationas well as at the exposure station, a second position sensor may beprovided to enable the positions of the substrate table to be tracked atboth stations, relative to reference frame RF.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

A metrology apparatus in the form of a scatterometer useful in anembodiment of the present invention, is shown in FIG. 3A. A targetgrating T and diffracted rays are illustrated in more detail in FIG. 3B.The dark field metrology apparatus may be a stand-alone device orincorporated in either the lithographic apparatus LA, e.g., at themeasurement station, or the lithographic cell LC. An optical axis, whichhas several branches throughout the apparatus, is represented by adotted line O. In this apparatus, light emitted by source 11 (e.g., axenon lamp) is directed onto substrate W via a beam splitter 15 by anoptical system comprising lenses 12, 14 and objective lens 16. Theselenses are arranged in a double sequence of a 4F arrangement. Therefore,the angular range at which the radiation is incident on the substratecan be selected by defining a spatial intensity distribution in a planethat presents the spatial spectrum of the substrate plane, here referredto as a (conjugate) pupil plane. In particular, this can be done byinserting an aperture plate 13 of suitable form between lenses 12 and14, in a plane which is a back-projected image of the objective lenspupil plane. Depending on the type of measurement being undertaken,different forms of aperture may be used. Examples might be a spot orannular aperture centered on the optical axis of the illumination systemformed by lenses 12, 14 and 16.

Using the annular aperture 13A, illustrated in FIG. 3C, the measurementbeam is incident on substrate W in a cone of angles not encompassing thenormal to the substrate. The illumination system thereby forms anoff-axis illumination mode with circular symmetry. Other modes ofillumination are possible by using different apertures, as will bedescribed. For the purpose of the measurements described herein,non-annular off-axis illumination modes are particularly suitable, suchas the aperture 13N illustrated in FIGS. 3A and C. This has an aperturein one or two quadrants of the conjugate pupil plan only, at an off-axisposition. The rest of the pupil plane is desirably dark as anyunnecessary light outside the desired illumination mode will interferewith the desired measurement signals.

As shown in FIG. 3B, target grating T is placed with substrate W normalto the optical axis O of objective lens 16. A ray of illumination Iimpinging on grating T from an angle off the axis O gives rise to azeroth order ray (solid line 0) and two first order rays (dot-chain line+1 and double dot-chain line −1). It should be remembered that, theserays are just one of many parallel rays covering the area of thesubstrate including metrology target grating T and possibly (with anoverfilled small target grating) other features. Since the annularaperture in plate 13 has a finite width (necessary to admit a usefulquantity of light, the incident rays I will in fact occupy a range ofangles, and the diffracted rays 0 and +1/−1 will be spread out somewhat.According to the point spread function of a small target, each order +1and −1 will be further spread over a range of angles, not a single idealray as shown.

At least the 0 and +1 orders diffracted by the target on substrate W arecollected by objective lens 16 and directed back through beam splitter15. Remembering that, when using the illustrated annular aperture plate13, incident rays I impinge on the target from a cone of directionsrotationally symmetric about axis O, first order rays −1 from theopposite side of the cone will also enter the objective lens 16, even ifthe ray −1 shown in FIG. 3B would be outside the aperture of objectivelens 16. Returning to FIG. 3A, this is illustrated by designatingdiametrically opposite portions of the annular aperture as north (N) andsouth (S). The +1 diffracted rays from the north portion of the cone ofillumination, which are labeled +1(N), enter the objective lens 16, andso do the −1 diffracted rays from the south portion of the cone (labeled−1(S)).

A second beam splitter 17 divides the diffracted beams into twomeasurement branches. In a first measurement branch, optical system 18forms a diffraction spectrum (pupil plane image) of the target on firstsensor 19 (e.g., a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for conventionalangle-resolved scatterometry on a range of different target types. Forthe present description, the purpose of the pupil image branch andsensor 19 is for measurement of overlay in large targets as part of amethod according to the present invention.

In the second measurement branch, optical system 20, 22 forms an imageof the target on the substrate W on sensor 23 (e.g., a CCD or CMOSsensor). In the second measurement branch, an aperture stop 21 isprovided in a plane that is conjugate to the pupil-plane. Aperture stop21 functions to block the zeroth order diffracted beam so that the imageof the target formed on sensor 23 is formed only from the first orderbeam. This is the so-called dark field image, equivalent to dark fieldmicroscopy. In embodiments of the present invention, this branch is usedfor dark field, image-based metrology on small, overfilled targets. Theimages captured by sensors 19 and 23 are output to image processor andcontroller PU, the function of which will depend on the particular typeof measurements being performed.

The particular forms of aperture plate 13 and field stop 21 shown inFIG. 3 are purely examples. In another embodiment of the presentinvention, on-axis illumination of the targets is used and an aperturestop with an off-axis aperture is used to pass substantially only onefirst order of diffracted light to the sensor. In yet other embodiments,2nd, 3rd and higher order beams (not shown in FIG. 3) can be used inmeasurements, instead of or in addition to the first order beams.

In yet other embodiments, apertures in stops 13 and/or 21 are notcircular or annular, but admit light at certain angles around theoptical axis only. Bipolar illumination can be used to form dark fieldimages of gratings aligned with the X and Y axes of substrate W.Depending on the layout of the apparatus, for example, illumination fromnorth and south poles may be used to measure a grating with linesparallel to the X axis, while illumination with east and west poles isused to measure a grating with lines parallel to the Y axis.

In order to make the illumination adaptable to these different types ofmeasurement, the aperture plate 13 may contain a number of aperturepatterns on a disc which rotates to bring a desired pattern into place.Alternatively or in addition, a set of plates 13 could be provided andswapped, to achieve the same effect. A programmable illumination devicesuch as a deformable mirror array can be used also. As just explained inrelation to aperture plate 13, the selection of diffraction orders forimaging can be achieved by altering the field stop 21, or bysubstituting a field stop having a different pattern, or by replacingthe fixed field stop with a programmable spatial light modulator. Whilethe optical system used for imaging in the present examples has a wideentrance pupil which is restricted by the field stop 21, in otherembodiments or applications the entrance pupil size of the imagingsystem itself may be small enough to restrict to the desired order, andthus serve also as the field stop.

FIG. 3C shows a set of aperture plates 13N, 13S, 13E, 13W which can beused to make asymmetry measurements of small target gratings, for thedark field overlay measurement method disclosed in our priorinternational patent application PCT/EP2010/060894 (applicant's refP3481.010), incorporated by reference herein in its entirety. Usingaperture plate 13N, for example, illumination is from north only, andonly the +1 order will pass through field stop 21 to be imaged on sensor23. By exchanging the aperture plate for plate 13S, then the −1 ordercan be imaged separately, allowing asymmetries in the target grating Tto be detected and analyzed. The same principle applies for measurementof an orthogonal grating and illuminating from east and west using theaperture plates 13E and 13W. The aperture plates 13N to 13W can beseparately formed and interchanged, or they may be a single apertureplate which can be rotated by 90, 180 or 270 degrees. As mentionedalready, the off-axis apertures illustrated in FIG. 3C could be providedin field stop 21 instead of in illumination aperture plate 13. In thatcase, the illumination could be on axis.

Different forms and applications of scatterometer are described forexample in US patent applications U.S. Pub. App. Nos. 2006/033921A andUS 2010/201963A, which are incorporated by reference herein in theirentireties. Application of such scatterometers to the measurement ofoverlay in composite gratings is described for example in US2006/0066855A, which is incorporated by reference herein in itsentirety. The entire content of these documents is hereby incorporatedby reference herein. In order to reduce the size of the targets, e.g.,to 10 μm by 10 μm or less, e.g., so they can be positioned in amongstproduct features, rather than in the scribe lane, so-called “smalltarget” metrology has been proposed, in which the grating is madesmaller than the measurement spot (i.e., the grating is overfilled).Typically small targets are measured using dark field scatterometry inwhich the zeroth order of diffraction (corresponding to a specularreflection) is blocked, and only higher orders processed. Examples ofdark field metrology can be found in international patent applicationsWO 2009/078708 and WO 2009/106279, which are incorporated by referenceherein in their entirety. Diffraction based overlay measurement by darkfield imaging is described in US patent application US 2010/328655A.These documents are hereby incorporated by reference herein in theirentirety. Other types of target and measurement, including image-basedmetrology are also available.

As an alternative, the substrate is rotated by 180°, rather thanrotating the illumination angle. This eliminates potential sources oferror, and the expense of throughput.

FIG. 4 depicts a composite target formed on a substrate according toknown practice. The composite target comprises four small gratings 32 to35 positioned closely together so that they will all be within themeasurement spot 31 formed by the illumination beam of the metrologyapparatus and thus are all simultaneously illuminated and simultaneouslyimaged on sensors 19 (pupil plane image) and 23 (substrate plane image).In an example dedicated to overlay measurement, gratings 32 to 35 arethemselves composite gratings formed by overlying gratings that arepatterned in different layers of the semi-conductor device formed onsubstrate W. Gratings 32 to 35 are differently biased in order tofacilitate measurement of overlay between the layers in which thedifferent parts of the composite gratings are formed. In one example,gratings 32 to 35 have biases of +d, −d, +3d, −3d respectively. Thismeans that one of the gratings has its components arranged so that ifthey were both printed exactly at their nominal locations one of thecomponents would be offset relative to the other by a distance d. Asecond grating has its components arranged so that if perfectly printedthere would be an offset of d but in the opposite direction to the firstgrating and so on. While four gratings are illustrated, a practicalembodiment might require a larger matrix to obtain the desired accuracy.

In addition to having different offsets among the gratings 32-35, thegratings may have different orientations, for example half of them beingoriented in the X direction and the other half in the Y direction. Forexample, X and Y gratings are schematically indicated in the targetshown in FIG. 4, the X-direction gratings 32, 34 may have offsets +d and−d respectively, while the Y-direction gratings 33, 35 also have offsetsof +d an d.

FIG. 5 shows examples of images that may be formed on and detected bythe sensor 23, using the target of FIG. 4 in the apparatus of FIG. 3,using the aperture plates 13N and the like from FIG. 3C. While the pupilplane image sensor 19 cannot resolve the different individual gratings32 to 35, the image sensor 23 can do so. The dark rectangle representsthe field of the image on the sensor, within which the illuminated spot31 on the substrate is imaged into a corresponding circular area 41.Within this, rectangular areas 42-45 represent the images of the smalltarget gratings 32 to 35. If the gratings are located in product areas,product features may also be visible in this image. Image processor andcontroller PU processes these images to identify the separate images 42to 45 of gratings 32 to 35. This can be done by pattern matchingtechniques, so that the images do not have to be aligned very preciselyat a specific location within the sensor frame. Reducing the need foraccurate alignment in this way greatly improves throughput of themeasuring apparatus as a whole.

Once the separate images of the gratings have been identified, theintensities of those individual images can be measured, e.g., byaveraging or summing selected pixel intensity values within theidentified areas Intensities and/or other properties of the images canbe compared with one another. Using different apertures at 13 and 21,different measurements can be taken. These results can be combined tomeasure different parameters of the lithographic process. Overlayperformance is an important example of such a parameter.

Using for example the method described in application PCT/EP2010/060894,which is incorporated by reference herein in its entirety, overlay errorbetween the two layers containing the component gratings 32 to 35 ismeasured through asymmetry of the gratings, as revealed by comparingtheir intensities in the +1 order and −1 order dark field images. Usingthe metrology apparatus of FIG. 3 with an aperture plate 13 having onlya single pole of illumination (e.g., north, using plate 13N), an imageof the gratings 32 to 35 is obtained using only one of the first orderdiffracted beams (say +1). Then, either the substrate W or the apertureplate 13 is rotated by 180° so that a second image of the gratings usingthe other first order diffracted beam can be obtained. For example, theaperture plate may be changed from 13N to 13S while keeping the opticalsystem otherwise the same. Consequently the −1(S) diffracted radiationis captured in the second image. FIG. 5A shows images 42(+) to 45(+) ofthe gratings 32-35, using only the +1 order diffracted radiation. FIG.5B shows images 42(−) to 45(−) of the same gratings, using only the −1order diffracted radiation. In embodiments where the substrate isrotated between measurements, the positions of the grating images willbe rotated also. The images A and B in FIG. 5 are generally similar, butwith different intensities of the grating images 42 to 45. Note that byincluding only half of the first order diffracted radiation in eachimage, the ‘images’ referred to here are not conventional dark fieldimages that would be produced using the aperture illustrated in FIG. 3A.The individual grating lines will not be resolved. Each grating will berepresented simply by an area of a certain grey level. The overlay canthen be determined by the image processor and controller PU by comparingthe intensity values obtained for +1 and −1 orders, and from knowledgeof the overlay biases of the gratings 32 to 35. As described in theprior application, X and Y direction measurements can be combined in oneillumination step by providing a first an aperture plate with, say,apertures at north and east portions, while a second aperture plate isprovided with apertures at south and west.

If the gratings are particularly close together on the substrate, it ispossible that the optical filtering it the second measurement branch maycause cross talk between signals. In that event, the central opening inthe spatial filter formed by field stop 21 should be made as large aspossible while still blocking the zeroth order.

It will be appreciated that the target arrays provided in thisembodiment of the present invention can be located in the scribe lane orwithin product areas. By including multiple targets within an areailluminated by the measurement spot 31 and imaged on sensor 23, severaladvantages may accrue. For example, throughput is increased byacquisition of multiple target images in one exposure, less area on thesubstrate need be dedicated to metrology targets and accuracy of overlaymeasurements can be improved, especially where there is a non-linearrelationship between the intensities of the different first orderdiffraction beams and overlay.

FIG. 6 shows an example of a larger overlay metrology target 50, whichis intended for measurement using the pupil plane image branch (sensor19) of the scatterometer, but can also be measured using the image-baseddark field branch (sensor 23). Target 50 again comprises a number ofgratings, of which one is labeled 52. The dashed circle 51 indicates anillumination spot, which may be the same spot as is labeled 31 in FIG.4. From this comparison, we see that the gratings 52 etc are largeenough to more than fill the illumination spot, so that there is noextraneous signal from product features, neighboring gratings etc. It isnot essential that the large targets are underfilled, however.

FIG. 7 at A and B illustrates images recorded by the dark field imagesensor 23, when illuminating the grating 52 with different illuminationmodes, so that the image at A records the pupil image 62 (+) using +1order radiation diffracted by target 52, and image B records pupil image62 (−) using −1 order diffracted radiation. As before, the order ofdiffracted radiation which is captured by the pupil image sensor can beselected by selecting different aperture plates, 13N, 13S etc. Also asbefore, the different diffraction orders can be selected by using asingle illumination mode, but rotating the substrate 180° betweenmeasurements. Again, each image A and B records a slightly differentintensity of radiation from the +1 and −1 orders. Comparison of theseintensities allows a measure of asymmetry in the gratings, and hence,with appropriate targets, overlay error. Again, different gratings 52etc within a large target may be provided with different biases in theiroverlay, so as to allow a qualitative estimate of overlay.

FIG. 7C illustrates an alternative method of measuring asymmetry, andhence overlay error, in grating 52, this time using the angle resolvedscatter spectrum detected by pupil image sensor 19. For this purpose,multiple measurements could be taken using apertures such as 13N and13S, or using a single aperture with different rotations of thesubstrate. In the example illustrated, however, a single angle resolvedscatter spectrum is imaged in the pupil plane image sensor 19, using theannular aperture 13A. In this single image, one can see the zero orderimage (reflection) of the aperture labeled 64(0). Also within the pupilimage on sensor 19 are portions 64(+) and 64(−), corresponding to the +1and −1 orders of diffracted radiation. Provided that the performance ofthe optical system is uniform and/or corrected with any necessarycalibrations, intensities of +1 and −1 order diffracted radiation fromtarget grating 52 can be compared by comparing the intensity of thesetwo portions of the pupil plane image measured by sensor 19. Asymmetryin these intensities carries information about asymmetry in the target,and hence overlay. As mentioned above, principles of diffraction-basedoverlay measurement are known from US patent application publication US20060066855A and 2010/0328655A1.

FIG. 8 shows schematically the overall layout of a patterning device M.As mentioned already, the metrology targets 72 may be included in ascribe lane portion of the applied pattern, between functional devicepattern areas 70. As is well known, patterning device M may contain asingle device pattern, or an array of device patterns if the field ofthe lithographic apparatus is large enough to accommodate them. Theexample in FIG. 8 A shows four device areas labeled D1 to D4. Scribelane targets 72 such as targets 800 and 800′ are placed adjacent thesedevice pattern areas and between them. On the finished substrate, suchas a semiconductor device, the substrate W will be diced into individualdevices by cutting along these scribe lanes, so that the presence of thetargets does not reduce the area available for functional devicepatterns. Where targets are small in comparison with conventionalmetrology targets, they may also be deployed within the device area, toallow closer monitoring of lithography and process performance acrossthe substrate. Some targets 74 of this type are shown in device area D1.While FIG. 8 shows the patterning device M, the same pattern isreproduced on the substrate W after the lithographic process, andconsequently this the description applies to the substrate W as well asthe patterning device.

Both the large target method of FIGS. 6 and 7, and the small targetmethod of FIGS. 4 and 5 are separately known for the measurement ofoverlay and/or other parameters in a lithography environment. Although,for example, the dark-field method with small targets is designed foroverlay measurements within the product areas 70, where the available‘real estate’ is very limited, it is expected to be less accurate andprecise than the diffraction based overlay method with large targets.This is due to a combination of factors, including the small size of thetarget area, the method of measurement and, in particular, a higherdependence on the overall lithography process than the diffraction-basedoverlay with large targets. With image-based overlay metrologyinstruments, not using dark-field imaging, similar problems will ariseas the target size shrinks. On the other hand, the larger targets canproduce more accurate overlay measurements with less process dependency,but occupy too great an area to be used with a high density inside theproduct area 70 of a commercial device manufacturing process. In orderto improve the performance and density of overlay measurements withoutsacrificing in-product real-estate, the scribe-lane marks 72 andin-product marks 74 of FIG. 8 are based on a hybrid of the large targetand small target systems, as will now be described.

FIG. 9 shows in more detail one of the product areas 70 on thepatterning device M, showing the targets 72 and 74 in more detail. Thesame pattern is produced and repeated at each field on the substrate.Product areas are labeled D and scribe-line areas are labeled SL. In thedevice areas 70, small targets 74 are spread with a desired density atdifferent locations among the product features. Theses targets have, forexample, the form illustrated in FIG. 4, and can be measured using thedark-field imaging sensor 23 of the scatterometer of FIG. 3. In thescribe-lane areas SL, large targets 72 a, for example of the typeillustrated in FIG. 6, are provided in a conventional manner. Besideeach of the large targets of 72 a is provided one or more small targets72 b, however, which may be in the scribe-lane areas SL or just insidethe product area 70. Where each target comprises a group of two or moreindividual gratings, the individual gratings of the small and largetargets could be positioned among the gratings of the large target,rather than being wholly separate. The small targets 72 b are identicalin form to the small targets 74 which are distributed over the productarea 70. With this combination of targets, the large scribe-lane targets72 a can be used to measure a lower order of model for overlay with highaccuracy, while the small in-product targets 74 can be measured withhigh density, and modeled as a perturbation of the lower order model.Since the regular targets 72 a are accompanied by smaller targets 72 bclose by, inaccuracies caused by, for example, process dependency in themeasured overlay using small targets can be known and compensated.

FIG. 10 illustrates graphically the novel, hybrid measurement conceptdescribed above. The horizontal axis X represents one dimension acrossthe substrate. The product area D and scribe-lane areas SL are delimitedon this axis. The vertical axis represents measured overlay values OVL.Two circled points labeled 82 a are plotted to show overlay valuesmeasured the large targets 72 a. Overlay values measured using the smalltargets 72 b adjacent to the large targets are labeled 82 b, and overlayvalues measured across the product area using the small targets 74 arelabeled 84. Applying, for the sake of example, a simple linear model tothe overlay across this product area, a profile curve 86 in singledot-dash line can be plotted between the points 82 a. This curve or canbe regarded as having a high accuracy where the overlay is sampled atpoints 82 a, but has no fine detail in the product area in between.Another profile curve 88 (double dot-dash line) is plotted from thesmall target measurements 82 b and 84. This higher order profileobviously contains far more detail in the X direction, but the itsmeasured overlay value is known to be subject to errors due to processdependency and limitations of the measuring instrument. Knowing that thetargets 72 b and 72 a are positioned substantially at the same point onthe substrate, however, an assumption can be made that the true overlayvalue represented by point 82 b on the graph is more accuratelyrepresented by the value of point 82 a. An offset 90 can therefore becalculated, and applied to all the points 84 so that the detail from thecurve 88 can be applied as a perturbation of the straight line 86, toobtain a curve 92, which has both absolute accuracy and detailedstructure across the product area.

While the simplified illustration of FIG. 10 shows only one dimension,the skilled reader will appreciate that the measurements and modelsextend in both the X and Y directions. Overlay in X and Y directions canalso be separately modeled across this two-dimensional area. Similarly,while the curve 86 is a linear model between just two sample points, areal substrate will have a number of measurements 82 a which can bejoined with a higher order model, again in two dimensions. Nevertheless,the addition of perturbations from the curve 88 allows yet higherorders, particularly showing in-product variations, to be added to themodel. The overlay values can be analyzed in a number of ways. Forexample, the processor can separate out perturbations which are commonto all fields of the substrate from those which vary across thesubstrate as a whole. Thus, an intra-field overlay fingerprint can beseparated from an inter-field overlay fingerprint.

The lower order model based on the large targets can, for example, be asix-parameter model, while the smaller targets are measured and modeledas a perturbation of one or more of the parameters, with third andhigher order terms. In one embodiment, the six parameters of overlayare: X, Y translation; symmetrical and asymmetric magnification;symmetric and asymmetric rotation. The lower order model contribution isassumed to be constant per image field, and is only linearlyinterpolated within the field, as shown in FIG. 10.

FIG. 11 is a flowchart showing the process for creating and measuring aparameter of a lithographic process, such as overlay. The process usesthe combination of large targets relatively sparsely distributed acrossthe substrate, and smaller targets that are more densely distributed. Atstep S1 a patterning device (reticle) or set of patterning devices isprovided with target patterns such as those illustrated in FIGS. 8 and 9distributed around and within product pattern areas 70. Where the targetis for measuring overlay, patterns for making the overlay metrologytarget will be included in at least two different reticles, which definepatterns in different layers of the semiconductor or other product.Where the patterning device is replaced by a programmable patterningdevice, the patterns are provided in data form, but the process isessentially the same. At S2, the metrology targets and product featuresare formed on the field areas of a substrate using a lithographicprocess. Of course a series of substrates will be patterned in practice,repeating step S2 and subsequent steps. The following steps areillustrated and described in a certain order, but can be performed invarious orders without departing from the principles of the process.

The patterned substrate is then loaded into a metrology apparatus suchas the scatterometer of FIG. 3. Steps S3 to S5 relate to measurement ofoverlay on the large targets 72 a. Using the FIG. 3 scatterometer,purely as an example, these measurements would be done with the pupilimage branch and sensor 19, as illustrated in FIGS. 6 and 7. At S3 andS4 the intensity of each target (and each grating within the target) ismeasured using only −1 order and only +1 diffracted radiation,respectively. At S5 the intensities are compared to derive a measure ofasymmetry, and hence overlay, in each target 72 a. At S6 the measuredlarge target overlay values are combined into a low-order profile ofoverlay across the substrate, represented by the curve 86 in FIG. 10.

At S7 and S8, the small targets 72 b and 74 across the substrate aremeasured with −1 order and +1 order diffracted radiation respectively.Using the FIG. 3 scatterometer, purely as an example, these measurementswould be done with the dark field imaging branch and sensor 23, asillustrated in FIGS. 4 and 5. For each target and each grating withinthe target, the two measured intensities are compared at step S9 toobtain an overlay measurement for each small target. At S10, these canbe combined to define a higher-order profile of overlay across thesubstrate, similar to curve 88 in FIG. 10.

At S11, the low order and higher order profiles from steps S6 and S11are merged, using the knowledge that small targets 72 b are adjacent tolarge targets 72 a, to produce a hybrid profile (i.e., curve 92 in FIG.10). The manner in which the data from the two types of target arecombined is not critical, and they may in fact be stored separately butwith cross-references and adjustments made, when they are used. Theprinciple is that the parameter (overlay for example) measured using thesmall targets can be corrected by reference to an offset observedbetween the small and large targets, which are adjacent one another.

As mentioned above, the profile of overlay and other parameters does notneed to be expressed entirely as a variation across the substrate. Itcan be expressed for example as an intrafield profile that is common toall fields (each instance of patterning using the patterning device M ata different location on the substrate W) and a lower order, interfield,variation onto which the intrafield variation is repeatedlysuperimposed.

Assuming that the profile of overlay errors does indeed have a strongintrafield component that is substantially the same for every field, ashortened measurement process can be implemented, as illustrated by thestep S12 in broken lines. In this modified embodiment of the process,the small targets 72 b, 74 are all measured only for a fewrepresentative fields, sufficient to determine and record the intrafieldprofile and the offset 90. The intrafield variations can then besuperimposed on the lower order interfield profile for all fields of thesubstrate, without measuring all the small targets across the substrate.Of course this embodiment assumes that the intrafield profile issufficiently repeating, which will need to be verified by experiment foreach process. The benefit is that the cost in time and measurementthroughput is not so great as in a process where all the small targetsare measured. Similarly, the correction to be applied between small andlarge target measurements may be constant across the substrate, andtherefore predictable from a few comparisons only. Alternatively,experiment may reveal that the correction is quite variable, and shoulditself be modeled as a parameter variable from field to field.

The measurements achieved by the embodiments described above can be usedpurely for research purposes, or they may be used to control productionof commercial devices. In the latter case, the process includes usingthe measured parameter profile to adjust parameters of the lithographyprocess when performed on subsequent substrates.

While specific embodiments of the present invention have been describedabove, it will be appreciated that the present invention may bepracticed otherwise than as described. In association with the physicalgrating structures of the novel targets as realized on substrates andpatterning devices, an embodiment may include a computer programcontaining one or more sequences of machine-readable instructionsdescribing a methods of producing targets on a substrate, measuringtargets on a substrate and/or analyzing measurements to obtaininformation about a lithographic process. This computer program may beexecuted for example within unit PU in the apparatus of FIG. 3 and/orthe control unit LACU of FIG. 2. There may also be provided a datastorage medium (e.g., semiconductor memory, magnetic or optical disk)having such a computer program stored therein.

Although specific reference may have been made above to the use ofembodiments of the present invention in the context of opticallithography, it will be appreciated that the present invention may beused in other applications, for example imprint lithography, and wherethe context allows, is not limited to optical lithography. In imprintlithography a topography in a patterning device defines the patterncreated on a substrate. The topography of the patterning device may bepressed into a layer of resist supplied to the substrate whereupon theresist is cured by applying electromagnetic radiation, heat, pressure ora combination thereof. The patterning device is moved out of the resistleaving a pattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the present invention that others can, byapplying knowledge within the skill of the art, readily modify and/oradapt for various applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1. A substrate comprising: product features formed on and distributedover the substrate, and a plurality of metrology targets adapted for usein measuring a parameter of performance of a lithographic process bywhich the product patterns have been applied to the substrate, whereinthe metrology targets include a first set of targets distributed atlocations across the substrate and a second set of targets which are formeasuring the same parameter, but which are smaller than the first set,wherein the second set of targets include a first subset distributed atlocations between the first set of targets, and a second subsetdistributed substantially at the same locations as the targets of thefirst set.
 2. The substrate of claim 1, wherein the parameter is overlayand each target is an overlay grating farmed in two patterned layers. 3.The substrate of claim 1, wherein each target is a composite targetcomprising a plurality of individual grating portions.
 4. The substrateof claim 1, wherein: the parameter is overlay and each target is anoverlay grating formed in two patterned layers, and different individualgrating portions are formed with different overlay biases.
 5. Thesubstrate of claim 1, wherein: the product features are arranged in aplurality of product areas separated by scribe lanes, and the targets ofthe first set are located primarily within the scribe lanes while thetargets of the second set are distributed within the product areas.
 6. Apatterning device for use in manufacturing a substrate, the patterningdevice comprising: product pattern features; and target patternfeatures, wherein the target pattern features are formed in first andsecond sets arranged so as to produce first and second sets of targetson the substrate when a pattern is applied from the patterning device tothe substrate.
 7. The patterning device of claim 6, wherein the targetpattern features are formed to produce an overlay grating.
 8. A methodof measuring a parameter of performance of a lithographic process bywhich product features have been applied to a substrate, the methodcomprising: simultaneously applying the product features and metrologytargets to the substrate, the metrology targets including a first set oftargets distributed at locations across the substrate and a second setof targets which are for measuring the same parameter but which aresmaller than the first set, the second set of targets including a firstsubset distributed at locations between the first set of targets, and asecond subset distributed substantially at the same locations as thetargets of the first set; illuminating the targets and detectingradiation diffracted or reflected by the targets and processing theradiation to determine values for the parameter at the locations of aplurality of the targets in each set; and correcting parameter valuesmeasured using the first subset of the second set of targets based on acomparison between values measured at one or more locations using atarget of the second subset and a target of the first set.
 9. The methodof claim 8, wherein the parameter is overlay and each target is formedby features in two patterned layers.
 10. The method of claim 8, whereinthe product features are arranged in product areas separated by scribelanes, wherein the targets of the first set are located primarily withinthe scribe lanes while the targets of the second set are distributedwithin product areas.
 11. The method of claim 8, wherein the targets aremeasured using a scatterometer having a first branch for performingangle-resolved scatterometry on the targets of the first set, and asecond branch for performing dark field imaging on the targets of thesecond set, the targets of the second set being overfilled by anillumination spot of the scatterometer.
 12. A device manufacturingmethod comprising: transferring a functional device pattern from apatterning device onto a substrate using a lithographic process whilesimultaneously transferring a metrology target pattern to the substrate;measuring the metrology target pattern to determine a value for one ormore parameters of the lithographic process; and applying a correctionin subsequent operations of the lithographic process in accordance withthe results of the metrology, wherein the metrology target patterncomprises a first set of targets distributed at locations across thesubstrate and a second set of targets which are for measuring the sameparameter but which are smaller than the first set, the second set oftargets including a first subset distributed at locations between thefirst set of targets, and a second subset distributed substantially atthe same locations as the targets of the first set.
 13. The devicemanufacturing method of claim 12, wherein the functional device patterncomprises product features arranged in a plurality of product areasseparated by scribe-lanes, and wherein the targets of the first set arelocated primarily within the scribe lanes while the targets of thesecond set are distributed within the product areas.